Why are these, and a great number of other international top leaders suddenly in Auckland,New Zealand?

Official reason:
They are all participating in the Pacific Islands Forum in order to discuss climate change

Real reason:New Zealand Foreign Minister Murray McCully said the forum's final day had been deliberately planned to coincide with the opening match of the Rugby World Cup as a way to attract more international guests to the Pacific summit.

"It was a conscious strategy to make this a magnet for big multilateral organisations whose presence could only enhance the standing of the forum but also provide practical support(read: money) for its initiatives," he told the NZ Herald.

PS
Already on his arrival in Auckland, Ban Ki-moon showed his knowledge of rugby by giving the following unforgettable statement:

"In rugby, you lose teeth. In diplomacy, you lose face"

EU Commission president Jose Manuel Barroso, who - like to other dignitaries - was welcomed by traditional songs and ukuleles, did not comment on the World Cup. But already before his arrival Barroso let it be known that his intention at the meeting is to announce even more "climate change" aid to the (not) "sinking" islands:

I will reemphasise the key role the EU intends to play as the region’s second largest donor to promote sustainable development and in helping to mitigate the impact of climate change in the region."